Prostate cancer is the second most commonly diagnosed cancer and the fourth leading cause of cancer-related death in men in the developed countries worldwide. Various prostate-associated antigens (PAA), such as prostate-specific antigen (PSA), prostate-specific membrane antigen (PSMA), and prostate stem cell antigen (PSCA) have been shown to be overexpressed by prostate cancer cells as compared to normal counterparts. These antigens, therefore, represent possible targets for inducing specific immune responses against cancers expressing the antigens via the use of vaccine-based immunotherapy. (See e.g. Marrari, A., M. Iero, et al. (2007). “Vaccination therapy in prostate cancer.” Cancer Immunol Immunother 56(4): 429-45)
PSCA is a 123-amino add membrane protein. The native full length human PSCA consists of amino adds 1 and 4-125 of SEQ ID NO:21 (without the alanine and serine residues at the second and third position respectively). PSCA has high tissue specificity and is expressed on more than 85% of prostate cancer specimens, with expression levels increasing with higher Gleason scores and androgen independence. It is expressed in 80-100% of bone metastasis of prostate cancer patients.
PSA is a kallikrein-like serine protease that is produced exclusively by the columnar epithelial cells lining the acini and ducts of the prostate gland. PSA mRNA is translated as an inactive 261-amino acid preproPSA precursor. PreproPSA has 24 additional residues that constitute the pre-region (the signal polypeptide) and the propolypeptide. Release of the propolypeptide results in the 237-amino acid, mature extracellular form, which is enzymatically active. The full length sequence of the native human PSA consists of amino acids 4-263 of SEQ ID NO: 15. PSA is organ-specific and, as a result, it is produced by the epithelial cells of benign prostatic hyperplastic (BPH) tissue, primary prostate cancer tissue, and metastatic prostate cancer tissue.
PSMA, also known as Folate hydrolase 1 (FOLH1), is composed of 750 amino acids. The amino acid sequence of the full length human PSMA is provided in SEQ ID NO:1. PSMA includes a cytoplasmic domain (amino acids 1-19), a transmembrane domain (amino acids 20-43), and an extracellular domain (amino acids 44-750). PSMA was found to be expressed in prostate cancer cells it at 1000-fold higher levels than normal tissues. It is abundantly expressed on neovasculature of a variety of other solid tumors such as colon, breast, liver, bladder, pancreas, lung, renal cancers as well as melanoma and sarcomas. Thus, PSMA is considered a target not only specific for prostate cancer cells but also a pan-carcinoma target for other cancers.
While a large number of tumor-associated antigens have been identified and many of these antigens have been explored as protein-based or DNA-based vaccines for the treatment or prevention of cancers, most clinical trials so far have failed to produce a therapeutic product. One of the challenges in developing cancer vaccines resides in the fact that the cancer antigens are usually self-derived and, therefore, poorly immunogenic because the immune system is self-regulated not to recognize self-proteins. Accordingly, a need exists for a method to enhance the immunogenicity or therapeutic effect of cancer vaccines.
Numerous approaches have been explored for enhancing the immunogenicity or enhancing anti-tumor efficacy of cancer vaccines. One of such approach involves the use of various immune modulators, such as TLR agonists, TNFR agonists, CTLA-4 inhibitors, and protein kinase inhibitors.
Toll-like receptors (TLRs) are type 1 membrane receptors that are expressed on hematopoietic and non-hematopoietic cells. At least 11 members have been identified in the TLR family. These receptors are characterized by their capacity to recognize pathogen-associated molecular patterns (PAMP) expressed by pathogenic organisms. These receptors in the innate immune systems exert control over the polarity of the ensuing acquired immune response. Among the TLRs, TLR9 has been extensively investigated for its functions in immune responses. Stimulation of the TLR9 receptor directs antigen-presenting cells (APCs) towards priming potent, TH1-dominated T-cell responses, by increasing the production of pro-inflammatory cytokines and the presentation of co-stimulatory molecules to T cells. CpG oligonucleotides, ligands for TLR9, were found to be a class of potent immunostimulatory factors. CpG therapy has been tested against a wide variety of tumor models in mice, and has consistently been shown to promote tumor inhibition or regression.
Cytotoxic T-Lymphocyte Antigen 4 (CTLA-4) is a member of the immunoglobulin superfamily and is expressed on the surface of Helper T cells. CTLA-4 is a negative regulator of CD28 dependent T cell activation, and acts as an inhibitory checkpoint for the adaptive immune response. Similar to the T-cell costimulatory protein CD28, CTLA-4 binds to CD80 and CD86 on antigen-presenting cells. CTLA-4 transmits an inhibitory signal to T cells, whereas CD28 transmits a stimulatory signal. Human antibodies against human CTLA-4 have been described as immunostimulation modulators in a number of disease conditions, such as treating or preventing viral and bacterial infection and for treating cancer (WO 01/14424 and WO 00/37504). Various preclinical studies have shown that CTLA-4 blockade by monoclonal antibodies enhances the host immune response against immunogenic tumors, and can even reject established tumors. Two fully human anti-human CTLA-4 monoclonal antibodies (mAbs), ipilimumab (MDX-010) and Tremelimumab (also known as CP-675206), have been investigated in clinical trials in the treatment of various types of solid tumors.
The tumor necrosis factor (TNF) superfamily is a group of cytokines that engage specific cognate cell surface receptors, the TNF receptor (TNFR) superfamily. Members of the tumor necrosis factor superfamily act through ligand-mediated trimerization, causing recruitment of several intracellular adaptors to activate multiple signal transduction pathways, such as apoptosis, NF-kB pathway, JNK pathway, as well as immune and inflammatory responses. Examples of the TNF superfamily include CD40 ligands, OX40 ligands, 4-1BB ligands, CD27, CD30 ligand (CD153), TNF-alpha, TNF-beta, RANK ligands, LT-alpha, LT-beta, GITR ligands, and LIGHT. The TNFR superfamily includes, for example, CD40, OX40, 4-1BB, CD70 (CD27 ligand), CD30, TNFR2, RANK, LT-beta R, HVEM, GITR, TROY, and RELT. Among the TNF members, CD40 agonists, including various CD40 agonistic antibodies, such as the fully human agonist CD40 monoclonal antibody CP870893, have been extensively explored for usage in therapies.
Protein kinases are a family of enzymes that catalyze the phosphorylation of specific residues in proteins. A number of kinase inhibitors have been investigated in clinical investigation for use in anti-cancer therapies, which includes, for example, MK0457, VX-680, ZD6474, MLN8054, AZD2171, SNS-032, PTK787/ZK222584, Sorafenib (BAY43-9006), SU5416, SU6668 AMG706, Zactima (ZD6474), MP-412, Dasatinib, CEP-701, (Lestaurtinib), XL647, XL999, Tykerb, (Lapatinib), MLN518, (formerly known as CT53518), PKC412, ST1571, AMN107, AEE 788, OSI-930, OSI-817, Sunitinib malate (Sutent; SU11248), Vatalanib (PTK787/ZK 222584), SNS-032, SNS-314 and Axitinib (AG-013736). Gefitinib and Erlotinib are two orally available EGFR-TKIs.